The invention relates to a process for the preparation of pulverulent metal oxides for ceramic compositions from solutions of the corresponding metal nitrates by evaporation of the solvent (water or dilute nitric acid) and thermal decomposition of the metal nitrate or of mixtures of metal nitrates by means of microwaves. Furthermore, the invention relates to an apparatus for carrying out the process.
The main purpose of the invention is to prepare metal oxide powders, in particular to prepare pulverulent mixtures of oxides of various metals, for further processing to give ceramic compositions for, for example, varistors, piezoelectrics, ceramic superconductors, soft/hard ferrites, capacitors, microwave filters, ceramic sensors, such as oxygen partial-pressure sensors, and the like.
As is known, the performance characteristics of the ceramics mentioned depend not only on the homogeneity of the ceramic structure but also, in the case of mixed oxide ceramics, very significantly on the homogeneity of the metal oxide mixture for the preparation of the ceramic. This is why the use of highly homogeneous, very fine and sinter-active powders or powder mixtures is indispensable for preparing ceramic structural components of the type mentioned.
A plurality of processes for preparing ceramic mixed oxide powders is known, the most important of which will be briefly outlined below.
In the so-called mixed oxide process, dry metal oxides and/or carbonates are mixed in the desired ratio of the composition of the ceramic to be prepared, and, if desired, ground and then calcined in a crucible or in a rotary kiln at temperatures below the sintering temperature of the desired ceramic. In order to improve the sintering activity of the powders obtainable by the mixed oxide process, a vigorous grinding process is often required. During this process, material rubbed off from the grinding bodies reaches the powder material, which may result in a deterioration of the mechanical and electrical properties of the ceramics obtained from the powders. In order to improve the homogeneity of the powder grain and powder mixture, it is in some cases necessary to repeat the grinding and calcining process several times. Nevertheless, the desired high-performance properties of the oxidic powders can be achieved only in a few cases.
More recently, the attempt has been made to counter the basic disadvantages of the preparation of ceramic mixed oxide powders from pulverulent substances by preparing the oxide powders from solutions or suspensions, for which the reaction spraying process, coprecipitation, the sol/gel process and the preparation of oxides or mixed oxides in a microwave field are used. The initial forms of the substrates are here preferably metal nitrates and metal alkoxides.
In the reaction spraying process, the mixture of the dissolved components is sprayed into a hot reaction space, in which the solution droplets are dried and converted to oxide particles while still in the air. Since delivery of energy to the droplets or particles only takes place slowly and from the surface, hollow spheres are usually formed in the spraying process (E. Ivers-Tiffee, K. Seitz "Characterization of Varistortype, Raw Materials Prepared by the Evaporative Decomposition of Solution Technique", American Ceramic Bulletin 66 [9], pages 1384-1388 (1987)). When a ceramic is prepared from a powder comprising hollow spheres, some of these hollow spaces are exposed after sintering upon investigation of the structure. Ceramics with pores of this type have significantly lower strength compared with tight-burned ceramics.
In the coprecipitation process, the hydroxides of the initially introduced metal salts are precipitated from a mixed salt solution by increasing the pH, usually by adding ammonia, and the hydroxide mixtures are processed further to give the mixed oxide powder. The difficulties of the process are the complicated process control (after extensive empirical preliminary experiments) in order to achieve simultaneous precipitation of the hydroxides of the different metal cations. In many cases, additional measures must be taken for the coprecipitation process, in order to obtain a sufficiently homogeneous composition of the hydroxide mixture precipitate, for example the addition of certain anions.
In contrast, the sol/gel process is an elegant and, in terms of the desired product properties, satisfactory method. In this process, mixtures of metal alcoholates are preferably slowly hydrolysed and then subjected to a polycondensation process which converts the hydrolysate into a gel, which is then calcined to give the metal oxide powder. The advantages of the sol/gel process are, if the process of alcoholate hydrolysis and polycondensation is carried out with appropriate care, the homogeneous composition of the gel and the small particle size and sintering activity of the metal oxide powders obtainable from the gel. However, a particular disadvantage are the high production costs, determined in particular by the raw materials cost, of metal oxide powders obtained in this manner. An operative disadvantage worth mentioning is the low solids content of the starting solutions and the gels as a result of the complex organic molecular radicals.
Finally, the preparation of metal oxides or metal oxides or metal oxide mixtures from the corresponding metal nitrates by thermal decomposition by means of microwaves, which is also the starting point for the present invention, is known, DE 32 32 867, J84 009 487, J59 079 179, DE 33 46 253, J55 104 926, J56 145 104. However, the known processes are mostly limited essentially to the one-component system manganese (for the production of dry-cell anodes), to the preparation of mixed oxides for glass- or ceramic-reinforced composites or structural components and to the preparation of mixed oxides of uranium and thorium (for the production of nuclear fuel elements), for which one example each is given, but the first two of which do not start from dissolved but from solid metal nitrate.
According to Japanese Patent No. 067,946, solid manganese nitrate (150 g) is subjected to pulsing microwave irradiation (2.45 GHz, 600 W) for 12 minutes (in each case, 15 s of irradiation time with pauses inbetween of 10 s) and, after a pause of 25 minutes, again subjected to this pulsing microwave irradiation for 2 minutes. For further processing, the manganese dioxide obtained is finely ground in a ball mill.
According to German Offenlegungsschrift 36 11 141, which relates to a mixture for preparing fracture-resistant, fiber-reinforced ceramic material by microwave heating, a fiber-reinforced fracture-resistant ceramic material is obtained by the following principle: inorganic fibers (SiC; Si.sub.3 N.sub.4), oxidic materials (glass; Al.sub.2 O.sub.3 ; ZrO.sub.2), a solid nitrate (NaNO.sub.3 ; Zr(NO.sub.3).sub.4) and a coupling agent (preferably glycerol) are mixed and subjected in a heat-resistant reaction vessel (diameter: 2.5 cm, height: 2cm) to microwave radiation (2.45 GHz) for 1 to 2 h, during which temperatures of around 1000.degree. C. are reached. The melt produced leads to a fractureresistant ceramic article. The long duration of the microwave irradiation serves, inter alia, to obtain an ordered spatial orientation of the reinforcing fibers.
According to U.S. Pat. No. 4,444,723, the oxidic materials are produced from solutions of plutonium nitrate or uranyl nitrate or mixtures of these nitrates in a microwave field by heating the solution to 100.degree. to 120.degree. C., evaporating the solvent until a moist nitrate cake is obtained, heating the nitrate cake to 350.degree. to 400.degree. C. for converting the metal nitrate into the metal oxide and then calcining the metal oxide at 700.degree. to 800.degree. C. The reaction set-up essentially comprises a circulating continuous belt carrying a plurality of open and rotating reaction dishes which pass through at least three microwave chambers arranged one after the other, in which the nitrate solution is first evaporated, the nitrate cake obtained, after a steep increase in temperature with constant temperature control and adjustment, is then denitrated to give the metal oxide and finally the metal oxide obtained is calcined to give the process product upon passing through a plurality of microwave chambers, likewise with constant temperature measurement and adjustment. The temperature of the irradiated material is adjusted by mechanical vertical movements of the reaction dishes moving horizontally and rotating on the continuous belt in the microwave field.
Although the conversion of metal nitrates to metal oxides in the microwave field is in principle known, the known processes cannot be utilised for preparing, in particular, mixed oxide powders for high-performance ceramics. Some of the reasons are as follows:
When starting from dry mixtures of the nitrates, fundamental disadvantages similar to those of the mixed oxide process described above have to be accepted. A further disadvantage, which can be derived from U.S. Pat. No. 4,444,723, is that nitrate materials initially introduced in dry form or obtained by evaporating a nitrate solution, after reaching a higher temperature and thus enhanced coupling properties with microwaves, have the tendency often to heat up rapidly and with glowing in an uncontrollable manner, which may lead to rapid sintering of the material. Owing to the regular inhomogeneities of the microwave field, possibly also owing to other causes, the glowing is often only limited locally. If the reaction is interrupted by switching off the microwave field and then continued, the same areas glow again, while the portions of the reaction material next to them take a different course of the reaction. As a result, the final products do not conform to the requirements of compositions or powders for preparing high-performance ceramics with respect to particle size homogeneity (if sintering to give more compact units has not already taken place) or with respect to a homogeneous distribution of the elements. Furthermore, the process products do not turn out to be nitrate-free with certainty
If, according to the fundamental teaching of U.S. Pat. No. 4,444,723, the starting materials are fairly large batches of mixed nitrate solutions, the comparatively slow evaporation of the mixed nitrate solution is bound to lead to partial separation of the components in the precipitate and to a mixed oxide product of accordingly inhomogeneous composition, due to the different solution properties of the individual metal nitrates. In the case of fairly large volumes of nitrate materials to be converted to the mixed oxide by means of microwaves, other factors causing separation are the different reaction temperatures and rates of decomposition which are site-dependent (on the surface and in the depth). In the case of partial sintering or even melting, further mixing and particle inhomogeneities occur.
It may also be mentioned that the reaction of solid metal nitrates encounters difficulties whenever the nitrate form is incapable of coupling with the incoming microwave irradiation at low temperatures, for example room temperature. An example of such a metal oxide is alumina. If nevertheless it is desired to decompose such a metal nitrate thermally by means of microwaves, the nitrate or mixed nitrate has to be heated to the coupling temperature either by means of a thermal energy source or a coupling agent (say glycerol, according to the teaching of German Offenlegungsschrift 36 11 141, or ammonium nitrate which decomposes without leaving a residue) has to be added. However, when starting from aqueous nitrate solutions, the situation where the metal nitrate or nitrate mixture to be reacted in the dry form by means of microwave irradiation has not yet reached the temperature of response for coupling with the microwaves can be avoided, except for a few cases. The reason for this is that water shows good coupling properties with microwaves, as a result of which an aqueous nitrate solution heats rapidly, the solvent can be readily evaporated and the metal nitrate obtained in crystalline form after evaporation of the solvent is present at temperatures of more than 100.degree. C., at which temperature most metal nitrates show good interaction with the microwaves.